Anisotropic Ferrimagnetism in Pyrrhotite – No Experiment is a Failure, Part One

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By Tim Raney…Bald Engineer Guy with Glasses

Introduction
The purpose of this experiment was to demonstrate the magnetocrystalline anisotropic ferrimagnetism sometimes shown by the iron sulfide mineral known as pyrrhotite. Magnetic anisotropy describes an effect where the magnetic properties of ferro- or ferrimagnetic materials show a preferred direction. I was interested in exploring this type of magnetic anisotropy since it occurs naturally based on the mineral’s chemical composition and associated crystal structure.

A similar incarnation of this demonstration experiment appears in Meiners, 1970[1]. Since I bought this two volume set years ago, I have wanted to try this demonstration. I also wanted to craft an experiment with a real hypothesis and maybe build impressive-looking apparatus too. This experiment appeared simple to execute. I just needed to buy a kilogram of pyrrhotite – more is better, right? Hang the specimen from a string near a magnet. I used nice, 100% nylon string too. Lastly, see what happens and record the stellar results and the extraordinary scientific insights. Well, as you might have expected, the experiment failed for a number of reasons. However, the insights gained through further study will assist me in refining the experiment and build better apparatus for future work. The apparatus might look really cool too, but that’s not the point, right?

Theory
The properties of magnetic materials (elements or compounds) can vary depending on the direction and magnitude of an applied magnetic field. For example, a given ferrimagnetic crystal is more easily magnetized along one crystallographic axis (easy direction) compared to another axis (hard direction). The difference in energy for these two states is the anisotropy energy[2]. There are several forms of anisotropy. My focus was on magnetocrystalline anisotropy – an intrinsic property of naturally occurring ferrimagnetic materials due to their crystal structure and independent of their mineral grain size or shape[3]. Anisotropism occurs in both ferro- and ferrimagnetic materials. Familiar examples of ferromagnetic materials include iron, nickel, cobalt and their alloys. Perhaps “ferrimagnetic materials” might not seem as familiar, but common examples include the mineral magnetite (Fe3O4), pyrrhotite (Fe1-xS) and ferrites (metal oxide + Fe2O3), a class of metallic oxides[4][5]. Thus, for the purpose this paper serves, we will digress here and discuss the difference between these two major classes of magnetic materials.

Ferromagnetism is an intrinsic property of iron, certain other elements, alloys and compounds as noted above. Some of the magnetic dipole moments are inherently aligned in these materials. An external magnetic field (Bext) can then align the magnetic moments further, producing a strong magnetic field. Moreover, this induced field can persist to some degree after removing Bext and results in a large net magnetization. This ferromagnetic behavior is due to a quantum physical effect called “exchange coupling” whereby the electron spins of one atom interact with other atoms nearby. This effect aligns the atom’s magnetic dipole moments despite the normal random atomic collisions, thus giving ferromagnetic materials their permanent magnetism[6].

Ferrimagnetic materials, often associated with ionic compounds, e.g., oxides, exhibit more complex forms of magnetic ordering due to their crystal structure. The simple diagram at right shows the magnetic dipole moments in a ferrimagnetic material represented by an oxide. Its structure has two magnetic sub-lattices (A and B) separated by oxygen ions. In this case, the oxygen anions mediate the atomic exchange forces/coupling in contrast to ferromagnetism[7]. END OF PART ONE.

Footnotes:

[1] H.F. Meiners (Ed.), Heat, Electricity & Magnetism, Optics, Atomic & Nuclear Physics, vol. II of Physics Demonstration Experiments, The Ronald Press Company, New York, 1970, pg. 969, sect. 32-3.12 and fig. 32-30.

[2] R.S. Elliot, Electromagnetics, McGraw-Hill Book Company, Inc., New York, 1966, pp. 443-444.

[3] B. M. Moskowitz, Hitchhiker’s Guide to Magnetism, Institute for Rock Magnetism, University of Minnesota, College of Science & Engineering, (http://www.irm.umn.edu/IRM/index.html). Downloaded 08 March 2012, pg. 17.

[4] R.S. Elliot, pg. 451.

[5] L.R. Moskowitz, Permanent Magnet Design and Application Handbook, Cahners Books International, Inc., Boston, MA, 1976, pg. 26.

[6] D. Halliday, R. Resnick and J. Walker, Fundamentals of Physics (6th Ed.), John Wiley & Sons, Inc., Hoboken, NJ 2003, pp. 752 and 755.

[7] B. M. Moskowitz, pg. 11.

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4 Responses to Anisotropic Ferrimagnetism in Pyrrhotite – No Experiment is a Failure, Part One

  1. Jim Hannon says:

    I can’t tell for sure but from looking at the picture of the sample of pyrrhotite it appears to be a concretion containing pyrrhotite. I would expect the anisotropy to be only observable in a sample that has most of the crystals oriented the same way. I would try something like cutting a flat slab of the pyrrhotite placing a piece of magnetic viewing film on the slab and putting it in a magnetic field that can be rotated.

  2. Dave says:

    Pyrrhotite is a rather strange mineral. It’s basically a version of Troilite, which is non-magnetic, only with a deficiency of Iron. The Iron deficiency results in the crystalline form of Pyrrhotite showing a magnetic behaviour. However, Pyrrhotite can crystallize in two different forms, monoclinic and hexagonal, with the hexagonal form having little magnetic properties.

    However, even after having said that, I see no evidence of crystalline structure in the sample you have photographed. It looks to be an amorphous lump (or a cementation). You might try cleaving the sample to see if exhibits any signs of crystalline behaviour upon cleaving.

    http://en.wikipedia.org/wiki/Cleavage_%28crystal%29

    The sample in this article shows signs of cleavage planes:

    http://en.wikipedia.org/wiki/Pyrrhotite

    Another interesting magnetic property to investigate is the Morin Transition in Hematite, which occurs at 260K (e.g., -13C or so).

    http://en.wikipedia.org/wiki/Morin_transition

    Dave

    P.S. In the magnetic work I’ve done, I’ve been a fan of the Allegro A1321 Hall Effect magnetic field sensor (<US$5), although I'm told that these have been replaced with a newer model. No connection with Allegro, other than as a satisfied customer.

  3. TIM RANEY says:

    Jim & Dave-

    Thanks very much for the excellent insights. The pyrrhotite sample shown is weathered quite a bit, but it’s not a concretion. It’s a massive form. I haven’t tried working a piece into a defined shape yet for further experiments.

    Thanks again for your comments!!!!

    TIM

  4. Pingback: Part 1: Rotatable Electromagnet Project | Citizen Scientists League

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